Fibrinogen is the precursor of fibrin, the material of which blood clots are composed. It is a large molecular weight complicated protein composed of two pairs of three non-identical polypeptide chains. A genuine X-ray structure has never been obtained. Now we are determining the structures of the principal fragments of human fibrinogen, the largest of which is called fragment D. We have collected data on crystals of this fragment out to 3.8 E on an area detector, and data from several isomorphous derivatives have provided a preliminary if somewhat ambiguous electron density map. We hope that synchrotron data would allow better resolution.